Field of the Invention
The invention relates to a method for flow measurement using a magnetic-inductive flowmeter with a measuring tube and a magnetic field generator, wherein a magnetic field is generated by the magnetic field generator and wherein the magnetic field is inverted at inverting intervals.
Description of Related Art
A magnetic-inductive flowmeter of this type measures the flow of a medium through the measuring tube, wherein the measuring tube is normally completely filled with the medium. For this, the magnetic field is generated by the magnetic field generator in such a manner that the magnetic field at least partially permeates the medium located in the measuring tube and a flow of the medium through the measuring tube induces an induction voltage in the medium. The induction voltage is thereby proportional to the magnetic flux density of the magnetic field in the medium. Normally, the magnetic field generator has an electromagnet with a coil for generating the magnetic field and with a yoke for guiding the magnetic field. The induction voltage is proportional to a flow velocity of the medium through the measuring tube and, from the flow velocity, a volume flow of the medium through the measuring tube is determined using e.g., an inner cross sectional area of the measuring tube or a mass flow of the medium through the measuring tube is determined additionally using a density of the medium. The flow of the medium through the measuring tube thus relates both to the volume flow as well as to the mass flow of the medium through the measuring tube.
Inversion of the magnetic field takes place, in each case, within one of the inverting intervals and corresponds to a reversal of the direction of the magnetic field. Accordingly, the strength of the magnetic field in the respective inverting interval is initially transient and then constant after settling. Such a magnetic field is also called a switched or a clocked constant magnetic field. When an electromagnet generates the magnetic field using a coil, a switched magnetic field is, e.g., generated in that a constant current is applied in the coil and the direction of the current is reversed after inverting intervals.
The flow velocity of a medium through the measuring tube is mostly different from zero, however can be zero, so that the medium is stagnant in the measuring tube. A measured flow for a medium stagnant in the measuring tube is also called zero flow. If a medium is stagnant in the measuring tube and if the measured zero flow is not equal to zero, the measured zero flow is also called zero flow error. A flow that is measured at a flow velocity different than zero is superimposed with the zero flow error, whereby the measuring accuracy of the flow is decreased.
A magnetic-inductive flowmeter is known from European Patent Application EP 1 970 675 A2 and corresponds U.S. Pat. No. 8,047,080 B2, which is designed to measure zero flows of a medium, to determine a deviation from subsequently measured zero flows and, if the deviation exceeds a predetermined value range, to indicate the exceedance. It is known from U.S. Pat. No. 5,402,685, to equip magnetic-inductive flowmeters with a calibrating and zero-balancing arrangement. U.S. Pat. No. 7,369,949 B2 proposes different methods for improving the measuring accuracy of magnetic-inductive flowmeters.
Methods for flow measurement are known from the prior art using a magnetic-inductive flowmeter of the generic kind, in the implementation of which, a flow of a medium through the measuring tube is measured and wherein the measured flow can deviate from the actual flow of the medium by a zero flow error that cannot be detected by the method.